Rear-view units of the generic type, in particular for commercial vehicles, as a rule have two servomotors for each mirror glass of an external mirror, in order to ensure adjustments about a horizontal and/or vertical positioning axis. The associated control switch or switches are in the driver's cabin or in the passenger cabin of the vehicle. In like manner the mirror glasses are usually provided with a heating which is equally actuated by an on-off switch from inside the driver's cabin. Finally, in some countries the housings of the external rear-view mirrors are provided with an additional lamp equally actuated by an on-off switch from the driver's cabin. These numerous electric consumers in the external rear-view mirror or in the external rear-view mirrors necessitate comparatively thick and expensive cables between the switches in the driver's cabin and the electric consumers situated outside the driver's cabin in the external rear-view mirror or mirrors. On the one hand this is rather costly, and on the other hand this complicates any subsequent equipment of a motor-vehicle with electrically adjustable external rear-view mirrors, when the vehicle had before only been provided with one or several manually adjustable, but heatable external rear-view mirrors.
So-called multiplex systems for motor-vehicles have been disclosed for instance by DE 25 06 073 C2. In this case a ring line to which a transmitting unit is associated is provided within a motor-vehicle. Each consumer is associated with a receiving unit. The instructions coded in the transmitting unit and entered via a plurality of actuating switches are decoded in the receiving unit connected in series before the associated consumer so that the corresponding consumer is triggered. Such multiplex systems only have one transmitting unit; but as a rule they have as many receiving units as consumers. The triggering of several consumers via one receiving unit is not possible. Further, a breakdown of the multiplex system will cause the electricity supply of the whole vehicle to collapse. Compared with this the electricity supply of the external rear-view mirrors of a motor-vehicle is of no substantial importance for the reliability of operation of the vehicle, since the external rear-view mirrors can always be adjusted manually. This possibility of manual adjustment is ensured by friction clutches generally provided for electrically actuatable external rear-view mirrors.